I'm creating a database system using SQLPlus which will be stored/connected to the cloud. I've been made aware that SQLPlus is not supported by Oracle Cloud and that I should use SQL Developer, is this true or will SQLPlus be supported by Oracle Cloud?

The Oracle Cloud is a broad term, but the Oracle Database Cloud Service does not support SQL*Net connections, which rules out SQL*Plus. There are much easier ways to create database objects with the Database Cloud Service,including SQL Developer and SQL Workshop in Application Express. Both of these tools will also run SQL scripts, which is how SQL*Plus works, if you would prefer that.

The upcoming DBaaS offering will include a dedicated database instance in a virtual machine, but that is not slated to be available for a while.